Electric charge will flow across a conductor when the two ends of the conductor are charged differently. This is known as potential difference (it is also known the the difference in voltage). Charge will no longer flow when the two ends have a common potential.
Ex: A wire is connected between a Van de Graff generator and the ground. There is a high potential between the two so charge will flow, but only for a little bit because the generator will quickly charge the ground, reaching a common potential. See image of Van de Graff generator below:
This image from: http://qedoc.org/en/index.php?title=Educational_Media_Awareness_Campaign/Physics/POTD_11
For flow to continue in a conductor there needs to be some arrangements made to keep one side at a higher potential then the other.
Ex: Water flowing through a reservoirs continues to flow because of a pump placed between the two to keep the pressures different.
34. 2 Electric Current
Electric current is the flow of electric charge. In a solid conductor, electrons carry the charge, while in gases and liquids, some positive ions also carry the charge.
Current is measured by the amount of charge that passes a certain place, this is known as an ampere (A or amp). Therefore one coulomb of charge in one second is one ampere.
Wires that carry electric charges do no have the electric charge themselves, the electrons inside the wires enter at one end and leave at the other at the same rate, keeping the electric charge consistant and maintaining the wires neutral charge.
34. 3 Voltage Sources
A voltage source is sometihng that provides a potential differnce between two points. Some examples of these include generators and batteries.
*This is analogous to the pump in the water reservoir.
34. 4 Electric resistance
Electric resistance is the property of conductors to resist the flow of electric charges and is measured in ohms. Longer or thinner wires have more resistance while shorter or fatter wires have less resistance. This is true because in shorter or fatter wires allow the electrons or charge to flow more easily. However, the resistance of the wire also depends on its conductivity. Metals with high conductivity such as aluminum or copper have a lower resistance.
34. 5 Ohm's Law
Ohm's Law states that :
the current in a circut (I) is equal to the voltage (V) divided by the resistance (R) therefore I = V/R
R = V/I
V= IR
This is a simple tool to help easily remember Ohm's Law To Use:
Place your finger over the value you are trying to find
The two remaining values will tell you what to do in order to find the other.
Ex: when trying to find voltage, your finger is placed over the V and you are left with multiplying I and R
34. 6 Ohm's Law and Electric Shock
When a potential difference is present across ones body a flow of electrons will occur. This flow can sometimes seriously injur someone, depending on the size of the current.
For very dry skin, your resistance is around 500,000 ohms. However, if you sweat (salt water) that resistance can drop to around 100 ohms.
A current of about .001 A is the least that can be felt. It can be painful around .005 A and fatal if it passes through your heart at .070 A
*Power supplies we use generally have small enough potential difference that usually (under normal conditions) they won't produce a dangerous current.
34. 7 Direct Current and Alternating Current
Direct Current: present generally in batteries, is when the electrons always flow in one direction. Alternating Current: which is what we get from our wall outlets, is when the electric current repeatedly reserves its direction, twice each cycle.
alternating current in wall outlets usually cycles at about 60 cycles per second, giving us a voltage of 120 volts.
alternating current is helpful when transporting electricity over long distanfces and then converting one voltage to another.
34. 8 Converting AC to DC
A device that will allow a current in one direction but block that of another is known as a diode.
This is used to convert AC to DC because it only allows one portion of the current to pass, or only a single direction of the current, which changes it to a pulsing DC.
34. 9 The Speed of Electrons in a Circuit
Individual electrons make it around a circuit at the drift speed, about 1 m every 3 hours.
In a completed circut, electrons being to flow immediately but electrons leaving a source do not actually travel through the circuit that quickly.
34. 10 The Source of Electrons in a Circuit
The electrons that flow through an electric circuit are already present in the circuit itself. It is the potential difference between the components of the circuit that make the electrons move.
34. 11 Electric Power
Electric Power is the electrical energ ythat is converted to heat as it flows through a circuit. Power is usually measured in watts. However, utility companies generally measures power in kilowatt-hours (kWh). A kilowatt-hour is the amount of energy consumed in 1 hour at a rate of 1 kilowatt.
Chapter 34 Electric Charge
34. 1 Flow of Charge
Electric charge will flow across a conductor when the two ends of the conductor are charged differently. This is known as potential difference (it is also known the the difference in voltage). Charge will no longer flow when the two ends have a common potential.Ex: A wire is connected between a Van de Graff generator and the ground. There is a high potential between the two so charge will flow, but only for a little bit because the generator will quickly charge the ground, reaching a common potential. See image of Van de Graff generator below:
This image from: http://qedoc.org/en/index.php?title=Educational_Media_Awareness_Campaign/Physics/POTD_11
For flow to continue in a conductor there needs to be some arrangements made to keep one side at a higher potential then the other.
Ex: Water flowing through a reservoirs continues to flow because of a pump placed between the two to keep the pressures different.
34. 2 Electric Current
Electric current is the flow of electric charge. In a solid conductor, electrons carry the charge, while in gases and liquids, some positive ions also carry the charge.Current is measured by the amount of charge that passes a certain place, this is known as an ampere (A or amp). Therefore one coulomb of charge in one second is one ampere.
34. 3 Voltage Sources
A voltage source is sometihng that provides a potential differnce between two points. Some examples of these include generators and batteries.*This is analogous to the pump in the water reservoir.
34. 4 Electric resistance
Electric resistance is the property of conductors to resist the flow of electric charges and is measured in ohms. Longer or thinner wires have more resistance while shorter or fatter wires have less resistance. This is true because in shorter or fatter wires allow the electrons or charge to flow more easily. However, the resistance of the wire also depends on its conductivity. Metals with high conductivity such as aluminum or copper have a lower resistance.34. 5 Ohm's Law
Ohm's Law states that :the current in a circut (I) is equal to the voltage (V) divided by the resistance (R) therefore
I = V/R
R = V/I
V= IR
This is a simple tool to help easily remember Ohm's Law
To Use:
Place your finger over the value you are trying to find
The two remaining values will tell you what to do in order to find the other.
Ex: when trying to find voltage, your finger is placed over the V and you are left with multiplying I and R
34. 6 Ohm's Law and Electric Shock
When a potential difference is present across ones body a flow of electrons will occur. This flow can sometimes seriously injur someone, depending on the size of the current.- For very dry skin, your resistance is around 500,000 ohms. However, if you sweat (salt water) that resistance can drop to around 100 ohms.
- A current of about .001 A is the least that can be felt. It can be painful around .005 A and fatal if it passes through your heart at .070 A
*Power supplies we use generally have small enough potential difference that usually (under normal conditions) they won't produce a dangerous current.34. 7 Direct Current and Alternating Current
Direct Current: present generally in batteries, is when the electrons always flow in one direction.Alternating Current: which is what we get from our wall outlets, is when the electric current repeatedly reserves its direction, twice each cycle.
34. 8 Converting AC to DC
A device that will allow a current in one direction but block that of another is known as a diode.This is used to convert AC to DC because it only allows one portion of the current to pass, or only a single direction of the current, which changes it to a pulsing DC.
34. 9 The Speed of Electrons in a Circuit
Individual electrons make it around a circuit at the drift speed, about 1 m every 3 hours.In a completed circut, electrons being to flow immediately but electrons leaving a source do not actually travel through the circuit that quickly.
34. 10 The Source of Electrons in a Circuit
The electrons that flow through an electric circuit are already present in the circuit itself. It is the potential difference between the components of the circuit that make the electrons move.34. 11 Electric Power
Electric Power is the electrical energ ythat is converted to heat as it flows through a circuit. Power is usually measured in watts. However, utility companies generally measures power in kilowatt-hours (kWh). A kilowatt-hour is the amount of energy consumed in 1 hour at a rate of 1 kilowatt.Power can be found by using this equation:
P = IV